Catch websocket error.

[sogaani]

The Gateway exits, when property change and websocket close occur at the same time.

[codecov-io]

Codecov Report

Merging #826 into master will decrease coverage by <.01%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master     #826      +/-   ##
==========================================
- Coverage   77.14%   77.13%   -0.01%     
==========================================
  Files          87       87              
  Lines        3763     3766       +3     
  Branches      501      502       +1     
==========================================
+ Hits         2903     2905       +2     
- Misses        788      789       +1     
  Partials       72       72

Impacted Files	Coverage Δ
src/controllers/things_controller.js	82.5% <83.33%> (-0.31%)	⬇️

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